Engine.get_singleton_list() does not include autoload scripts defined in the project settings.

### Tested versions

v4.2.2.stable.custom_build [0cfe9491e]

### System information

Windows 10

### Issue description

[Autoload Scripts are also called Singletons in the docs](https://docs.godotengine.org/en/stable/tutorials/scripting/singletons_autoload.html) but I do not see them in `Engine.get_singleton_list()` thus I cannot get them with `Engine.get_singleton()` when I probably should be able to.

I only see the inbuilt singletons when I print the singleton list:
```
["Performance", "TextServerManager", "PhysicsServer2DManager", "PhysicsServer3DManager", "NavigationMeshGenerator", "ProjectSettings", "IP", "Geometry2D", "Geometry3D", "ResourceLoader", "ResourceSaver", "OS", "Engine", "ClassDB", "Marshalls", "TranslationServer", "Input", "InputMap", "EngineDebugger", "Time", "GDExtensionManager", "ResourceUID", "WorkerThreadPool", "ThemeDB", "JavaClassWrapper", "JavaScriptBridge", "DisplayServer", "RenderingServer", "AudioServer", "PhysicsServer2D", "PhysicsServer3D", "NavigationServer2D", "NavigationServer3D", "XRServer", "CameraServer"]
```

### Steps to reproduce

Create some Autoload Scripts in the project settings and see if they show up in `Engine.get_singleton_list()`.

### Minimal reproduction project (MRP)

N/A

Start by reproducing the issue with an autoload configured in Project Settings, then inspect the entry points named in the report: Engine.get_singleton_list() and Engine.get_singleton(). Done means configured autoload scripts appear in the singleton list and can be retrieved through the singleton accessor, without removing the built-in entries.

Written by the indexing model from the issue text.